Control devices, e.g. two-wire line voltage electronic thermostats, are commonly used to control the operation of electric heating systems, such as baseboard heaters, radiant floors, radiant ceilings, convectors, or the like. Such thermostats may comprise a triode alternating current switch (or TRIAC) placed in series with a load (e.g. the heating system) for controlling the current flowing in the load. In response to an input, the circuit is closed to connect the load to an alternating current (AC) power source. This leads to a voltage drop across the terminals of the thermostat. However, it is desirable for the thermostat to remain operational while the load is being fed. Thus, in order to operate the thermostat, there is a need for some of the source's power to be diverted by the internal circuitry of the thermostat, and this without disturbing the load.
For this purpose, a common technique is to delay the triggering of the TRIAC so that the voltage generated across the terminals is stored in an accumulator. Once the accumulated energy is sufficient to operate the thermostat's internal circuitry for the remaining cycle time, the TRIAC is triggered. A disadvantage of this technique is that the higher the power required by the thermostat's internal circuitry, the longer the TRIAC triggering delay for ensuring that sufficient energy is accumulated, the higher the voltage across the TRIAC at the time of commutation, and the higher the voltage rate of change (dv/dt), the harmonics, and the EMI generated on the power line. Although such interference may be eliminated using passive filters for small loads, this may not prove suitable for large loads as bulky windings and capacitors would be required.
This is especially true for modern thermostats, which are provided with radiofrequency (RF) communication functionalities that come with increased power requirements.
There is therefore a need for an improved power stealing circuit for a control device.